Mn‐Cr isotope systematics of the D'Orbigny angrite

Abstract— We have conducted a detailed study of the Mn‐Cr systematics of the angrite D'Orbigny. Here, we report Cr isotopic abundances and Mn/Cr ratios in olivine, pyroxene, glass, chromite, and bulk rock samples from D'Orbigny. ⁵³Cr excesses in these samples correlate well with their respective Mn/Cr ratios and define an isochron with a slope that corresponds to an initial ⁵³Mn/⁵⁵Mn ratio = (3.24 ± 0.04) × 10^?6 and initial ⁵³Cr/⁵²Cr ratio of ?(53) = 0.30 ± 0.03 at the time of isotopic closure. The ⁵³Mn/⁵⁵Mn ratio of the D'Orbigny bulk rock is more than two‐fold the ⁵³Mn/⁵⁵Mn ratio of the angrites Lewis Cliff 86010 (LEW) and Angra dos Reis (ADOR) and implies an older Mn‐Cr age of 4562.9 ± 0.6 Ma for D'Orbigny relative to a Pb‐Pb age of 4557.8 ± 0.5 Ma for LEW and ADOR. One of the most unusual aspects of D'Orbigny is the presence of glass, a phase that has not been identified in any of the other angrites. The Mn‐Cr data for glass and a pyroxene fraction found in druses indicate that they formed contemporaneously with the main phases of the meteorite. Since the Mn‐Cr age of D'Orbigny is ?5 Ma years older than the angrites LEW and ADOR, D'Orbigny likely represents an earlier stage in the evolution of the angrite parent body.     

Petrology and geochemistry of D'Orbigny,geochemistry of Sahara 99555, and the origin of angrites

Abstract— We have done a detailed petrologic study of the angrite, D'Orbigny, and geochemical study of it and Sahara 99555. D'Orbigny is an igneous‐textured rock composed of Ca‐rich olivine, Al‐Ti‐diopside‐hedenbergite, subcalcic kirschsteinite, two generations of hercynitic spinel and anorthite, with the mesostasis phases ulvöspinel, Ca‐phosphate, a silico‐phosphate phase and Fe‐sulfide. We report an unknown Fe‐Ca‐Al‐Ti‐silicate phase in the mesostasis not previously found in angrites. One hercynitic spinel is a large, rounded homogeneous grain of a different composition than the euhedral and zoned grains. We believe the former is a xenocryst, the first such described from angrites. The mafic phases are highly zoned; mg# of cores for olivine are ?64, and for clinopyroxene ?58, and both are zoned to Mg‐free rims. The Ca content of olivine increases with decreasing mg#, until olivine with ?20 mol% Ca is overgrown by subcalcic kirschsteinite with about 30–35 mol% Ca. Detailed zoning sequences in olivine‐subcalcic kirschsteinite and clinopyroxene show slight compositional reversals. There is no mineralogic control that can explain these reversals, and we believe they were likely caused by local additions of more primitive melt during crystallization of D'Orbigny. D'Orbigny is the most ferroan angrite with a bulk rock mg# of 32. Compositionally, it is virtually identical to Sahara 99555; they are the first set of compositionally identical angrites. Comparison with the other angrites shows that there is no simple petrogenetic sequence, partial melting with or without fractional crystallization, that can explain the angrite suite. Angra dos Reis remains an anomalous angrite. Angrites show no evidence for the brecciation, shock, impact metamorphism, or thermal metamorphism that affected the howardite, eucrite, diogenite (HED) suite and ordinary chondrites. This suggests that the angrite parent body may have followed a fundamentally different evolutionary path than did these other parent bodies.     

Spectral properties of angrites

Abstract— Angrites are generally believed to be fragments of a basaltic asteroid that differentiated under relatively oxidizing conditions. Almost all angrites (e.g., D'Orbigny, Lewis Cliff [LEW] 86010, and Sahara 99555) are composed predominately of anorthite, Al‐Ti diopside‐hedenbergite, and Ca‐rich olivine, except for the type specimen, Angra dos Reis, which is composed almost entirely of Al‐Ti diopside‐hedenbergite. D'Orbigny, LEW 86010, and Sahara 99555 also have spectral properties very different from Angra dos Reis. These newly measured angrites all have broad absorption features centered near 1 μm with very weak to absent absorption bands at ?2 μm, which is characteristic of some clinopyroxenes. The spectrum of Angra dos Reis has the characteristic 1 and 2 μm features due to pyroxene. One asteroid, 3819 Robinson, has similar spectral properties to the newly measured angrites in the visible wavelength region, but does not appear to spectrally match these angrites in the near‐infrared.     

Stable isotope analysis of carbon and nitrogen in angrites

Angrites are a small group of ancient basaltic achondrites, notable for their unusual chemistry and extreme volatile depletion. No comprehensive study of indigenous light elements currently exists for the group. Measurement of the abundances and isotopic composition of carbon and nitrogen could provide information pertaining to the evolution of the angrite parent body. Bulk‐sample stepped combustion analyses of five angrites and a glass separate from D'Orbigny were combined with earlier data and acid dissolution experiments of carbonates found in D'Orbigny to compile an inventory of indigenous carbon and nitrogen. Indigenous carbon combusted between 700 °C and 1200 °C, with abundances of 10–140 ppm and a mass‐weighted δ¹³C of ?25 to ?20‰ with the exception of D'Orbigny (δ¹³C approximately ?5‰). Nitrogen was released at 850–1200 ºC, 1–20 ppm with a δ¹⁵N ?3‰ to +4‰; again, D'Orbigny (δ¹⁵N approximately +20 to +25‰) was an exception. We interpret these components as largely indigenous and decoupled; the carbon in graphitic or amorphous form, while the nitrogen is present as a dissolved component in the silicates. No relationship with the textural sub‐classification of angrites is apparent. We suggest that the angrite parent body contains a reservoir of reduced carbon and thus may have undergone a change in redox conditions, although the timing and mechanism for this remain unclear.     

Thermal histories of angrite meteorites: Trace element partitioning among silicate minerals in Angra dos Reis,Lewis Cliff 86010, and experimental analogs

Abstract— We measured rare earth element (REE) abundances in selected silicate phases in the angrites Angra dos Reis (AdoR) and Lewis Cliff (LEW) 86010 in order to further clarify the thermal history of AdoR. We also carried out a preliminary experimental study designed to examine apparent REE partitioning between silicates (fassaite, olivine, kirschsteinite, and melt) in synthetic analogs of angrites under disequilibrium conditions at liquidus temperatures. Silicates in AdoR are homogeneous with respect to major, minor, and trace elements, which is consistent with the interpretation that AdoR underwent extensive subsolidus equilibration. REE distributions in olivine and kirschsteinite in AdoR are similar to those in LEW 86010 and are consistent with the formation of kirschsteinite by exsolution from olivine during cooling and/or annealing. There is no evidence for a disequilibrium trace element signature that could have been inherited from rapid cooling at liquidus temperatures. This is supported by our petrographic observations of the occurrence of kirschsteinite within olivine aggregates in AdoR. Olivine/kirschsteinite pairs in AdoR record closure temperatures around 600–650 °C.     

Glass‐bearing inclusions in Shergotty and Chassigny: Consistent samples of a primary trapped melt?

Glass‐bearing inclusions hosted by different mineral phases in SNC meteorites provide important information on the conditions that prevailed during formation of early phases and/or on the composition of the primary trapped liquids/melts of these rocks. Although extensive previous work has been reported on such inclusions, several questions are still unresolved. We performed a chemical and petrographic study of the constituents (glasses and mineral assemblage) of glassy and multiphase inclusions in Shergotty and Chassigny. We focused on obtaining accurate trace element contents of glasses and co‐existing minerals and discussing their highly variable REE contents. Our results reveal an unusual geochemistry of trace element contents that appear to be independent of their major element compositions. Chemical equilibrium between phases inside inclusions as well as between glasses and host minerals could not be established. The LREE contents of glasses in glass inclusions can vary by up to two orders of magnitude. The depletion in trace element abundances shown by glasses seem to be inconsistent with these phases being residual melts. The light lithophile element contents of glasses are highly variable with enrichment in incompatible elements (e.g., Be, Sr, Ba, and LREE) indicating some processes involving percolation of fluids. All of these features are incompatible with glass‐bearing inclusions in the host minerals acting as closed systems preserving unmodified primary liquids/melts. Glass‐bearing inclusions in Shergotty and Chassigny appear to have been altered (as was the rock itself) by different postformational processes (e.g., shock, metamorphism, metasomatic [?] fluids) that affected these meteorites with different degree of intensity. Our results indicate that these inclusions could not preserve a reliable sample of the primary trapped melt.     

Chemical characterization of a unique chondrite: Allan Hills 85085

Abstract— Allan Hills 85085 is a new and very important addition to the growing list of unique carbonaceous chondrites because of its unique chemical and mineralogical properties. Previously published data on ALH85085 have established its bulk composition, gross levels of volatile depletion, siderophile enrichment, and the unique character of this unique meteorite. This chemical study provides more precise data on the major, minor, and trace element characteristics of ALH85085. ALH85085 has compositional, petrological, and isotopic affinities to Al Rais and Renazzo, and to Bencubbin-Weatherford. The similarities to Al Rais and Renazzo, in particular, suggest similar formation locations and thermal processing, possibly in the vicinity of CI chondrites. Petrologic, compositional and isotopic studies indicate that the components that control the abundance of the various refractory and volatile elements were not allowed to equilibrate with the nebula as conditions changed. This lack of equilibration could explain some of the compositional and petrologic variations within and between these meteorites and the inconsistencies in the classification of these meteorites using known taxonomic parameters. It is also apparent that the compositional trends implied by the application of taxonomic parameters probably did not vary simply with heliocentric distance. In addition, variations in the agglomeration of diverse nebular phases along with local and regional temperature events may strongly influence the bulk composition of meteorites.     

Accretional and alterational differences in a carbonaceous chondrite parent body: Evidence from the NWA 5491 CV3 meteorite

The NWA 5491 CV3 meteorite is a CVoxA subtype, and composed of two substantially different units (titled “upper” and “lower” units) in the cm size range with original accreted material and also subsequent alteration produced features. Based on the large chondrules in the “upper” unit and the small chondrules plus CAIs in the “lower” unit, they possibly accreted material from different parts of the solar nebula and/or at different times, whereas substantial changes happened in the nebula's composition. Differences are observed in the level of early fragmentation too, which was stronger in the upper units. During later alteration oxidizing fluids possibly circulated only in the upper unit, mechanical fragmentation and resorption were also stronger there. In the last phase of the geological history these two rock units came into physical contact, but impact‐driven shock effects were not observed. The characteristics of this meteorite provide evidence that the same parent body might accrete substantially different material and also the later processes could differ spatially in the parent body.     

The Tucson ungrouped iron meteorite and its relationship to chondrites

Abstract– Tucson is an enigmatic ataxitic iron meteorite, an assemblage of reduced silicates embedded in Fe‐Ni metal with dissolved Si and Cr. Both, silicates and metal, contain a record of formation at high temperature (～1800 K) and fast cooling. The latter resulted in the preservation of abundant glasses, Al‐rich pyroxenes, brezinaite, and fine‐grained metal. Our chemical and petrographic studies of all phases (minerals and glasses) indicate that they have a nebular rather than an igneous origin and give support to a chondritic connection as suggested by Prinz et al. (1987) . All silicate phases in Tucson apparently grew from a liquid that had refractory trace elements at approximately 6–20 × CI abundances with nonfractionated (solar) pattern, except for Sc, which was depleted (～1 × CI). Metal seems to have precipitated before and throughout silicate aggregate formation, allowing preservation of all evolutionary steps of the silicates by separating them from the environment. In contrast to most chondrites, Tucson documents coprecipitation of metal and silicates from the solar nebula gas and precipitation of metal before silicates—in accordance with theoretical condensation calculations for high‐pressure solar nebula gas. We suggest that Tucson is the most metal‐rich and volatile‐element‐poor member of the CR chondrite clan.     

Neodymium,strontium and chromium isotopic studies of the LEW86010 and Angra dos Reis meteorites and the chronology of the angrite parent body

Abstract— Neodymium, strontium, and chromium isotopic studies of the LEW86010 angrite established its absolute age and the formation interval between its crystallization and condensation of Allende CAIs from the solar nebula. Pyroxene and phosphate were found to contain ～98% of its Sm and Nd inventory. A conventional ¹⁴⁷Sm-¹⁴³Nd isochron yielded an age of 4.53 ± 0.04 Ga (2 σ) and ?¹⁴³ _Nd = 0.45 ± 1.1. An ¹⁴⁶Sm-¹⁴²Nd isochron gives initial ¹⁴⁶Sm/¹⁴⁴Sm = 0.0076 ± 0.0009 and ?¹⁴³ _Nd = ?2.5 ± 0.4. The Rb-Sr analyses give initial ⁸⁷Sr/⁸⁶Sr (I⁸⁷_Sr) = 0.698972 ± 8 and 0.698970 ± 18 for LEW and ADOR, respectively, relative to ⁸⁷Sr/⁸⁶Sr = 0.71025 for NBS987. The difference, ΔI⁸⁷_Sr, between I⁸⁷_Sr for the angrites and literature values for Allende CAIs, corresponds to ～9 Ma of growth in a solar nebula with a CI chondrite value of ⁸⁷Rb/⁸⁶Sr = 0.91, or ～5 Ma in a nebula with solar photospheric ⁸⁷Rb/⁸⁶Sr = 1.51. Excess ⁵³Cr from extinct ⁵³Mn (t1/2 = 3.7 Ma) in LEW86010 corresponds to initial ⁵³Mn/⁵⁵Mn = 1.44 ± 0.07 × 10^?6 and closure to Cr isotopic homogenization 18.2 ± 1.7 Ma after formation of Allende inclusions, assuming initial ⁵³Mn/⁵⁵Mn = 4.4 ± 1.0 × 10^?5 for the inclusions as previously reported by the Paris group (Birck and Allegre, 1988). The ¹⁴⁶Sm/¹⁴⁴Sm value found for LEW86010 corresponds to solar system initial (¹⁴⁶Sm/¹⁴⁴Sm)_o = 0.0080 ± 0.0009 for crystallization 8 Ma after Allende, the difference between Pb-Pb ages of angrites and Allende, or 0.0086 ± 0.0009 for crystallization 18 Ma after Allende, using the Mn-Cr formation interval. The isotopic data are discussed in the context of a model in which an undifferentiated “chondritic” parent body formed from the solar nebula ～2 Ma after Allende CAIs and subsequently underwent differentiation accompanied by loss of volatiles. Parent bodies with Rb/Sr similar to that of CI, CM, or CO chondrites could satisfy the Cr and Sr isotopic systematics. If the angrite parent body had Rb/Sr similar to that of CV meteorites, it would have to form slightly later, ～2.6 Ma after the CAIs, to satisfy the Sr and Cr isotopic systematics.     

REE abundances in the matrix of the Allende (CV) meteorite: Implications for matrix origin

Abstract— The trace element distributions in the matrix of primitive chondrites were examined using four least‐contaminated matrix specimens from the polished sections of the Allende (CV) meteorite. Analysis of rare earth element (REE), Ba, Sr, Rb, and K abundances by isotope dilution mass spectrometry revealed that the elemental abundances of lithophile elements except for alkali metals (K, Rb) in the specimens of the Allende matrix studied here are nearly CI (carbonaceous Orgueil) chondritic (～1 × CI). Compared to refractory elements, all the matrix samples exhibited systematic depletion of the moderately volatile elements K and Rb (0.1–0.5 × CI). We suggest that the matrix precursor material did not carry significant amounts of alkali metals or that the alkalis were removed from the matrix precursor material during the parent body process and/or before matrix formation and accretion. The matrix specimens displayed slightly fractionated REE abundance patterns with positive Ce anomalies (CI‐normalized La/Yb ratio = 1.32–1.65; Ce/Ce* = 1.16–1.28; Eu/Eu* = 0.98–1.10). The REE features of the Allende matrix do not indicate a direct relationship with chondrules or calcium‐aluminum‐rich inclusions (CAIs), which in turn suggests that the matrix was not formed from materials produced by the breakage and disaggregation of the chondrules or CAIs. Therefore, we infer that the Allende matrix retains the REE features acquired during the condensation process in the nebula gas.     

The origin of eucrites,diogenites, and olivine diogenites: Magma ocean crystallization and shallow magma chamber processes on Vesta

The asteroid 4 Vesta is one of the very few heavenly bodies to have been linked to samples on Earth: the howardite‐eucrite‐diogenite (HED) meteorite suite. This large and diverse suite of meteorites provides a detailed picture of Vesta's igneous and postigneous history. We have used the range of igneous rock types and compositions in the HED suite to test a series of chemical models for solidification processes following peak melting (magma ocean) conditions on Vesta. Fractional crystallization cannot have been a dominant early process in the magma ocean because it leads to excessive Fe‐enrichment in the melt. Models that are dominated by equilibrium crystallization cannot produce orthopyroxene cumulates (diogenites). Our best models invoke 60–70% equilibrium crystallization of a magma ocean followed by continuous extraction of the residual melt into shallow magma chambers. Fractional crystallization in these magma chambers combined with continuous or periodic addition of more melt from the slowly compacting crystal mush (magmatic recharge) can produce all of the igneous HED lithologies (noncumulate and cumulate eucrites, diogenites, dunites, harzburgites, and olivine diogenites). Magmatic recharge can also explain the narrow range in eucrite compositions and the variability of incompatible trace element concentrations in diogenites. We predict an internal structure for Vesta that permits excavation of the HEDs during the formation of the Rheasilvia basin, while remaining consistent with observations from the Dawn mission and most impact models.     

Aqueous alteration of carbonaceous chondrites: Evidence for preaccretionary alteration—A review

Abstract— In most groups of carbonaceous chondrites, minerals occur that are formed due to aqueous alteration in the nebula and/or within meteorite parent bodies. For determining the evolution of materials in the early solar system, it is of significant importance to clearly identify evidence for either nebular or planetary aqueous alteration. Therefore, results from the study of chondrites have fundamental implications for ideas concerning nebular dynamics, gas-solid interactions in the nebula, and accretionary processes. Considering the topic of this review, it is important to define nebular and parent body processes. The solar nebula activity should include condensation processes of high- and low-temperature components as well as processes of chemical fractionation and grain-size sorting, mixing of solids and gas, and interactions between early formed solids with the remaining gas; but it should exclude processes that occurred in small uncompacted protoplanetary objects that may have been totally destroyed again before accretion of the final meteorite parent bodies. Therefore, the term “preaccretionary” instead of “nebula” is used in this paper to include all these processes that may have occurred in small precursor planetesimals. Currently, there is no doubt that parent body aqueous alteration is a fundamental process in the evolution of several groups of carbonaceous chondrites. However, due to textural and mineralogical observations and chemical analyses, strong arguments have been found also indicating preaccretionary aqueous alteration of distinct components in carbonaceous chondrites. In this paper, evidence for preaccretionary aqueous alteration in carbonaceous chondrites and their components taken from previous studies is discussed in detail. The strongest evidence for preaccretionary alteration comes from studies of CM chondrites. The survival of highly unequilibrated mineral assemblages in accretionary rims, and sharp contacts between chondrule glass and surrounding phyllosilicates, are only two important arguments for preaccretionary alteration features among many others discussed in the paper. Similar observations were also made in CR, CH and related chondrites. Due to the small abundance of water-bearing phases in CO and CV chondrites, the origin of phyllosilicates in these groups is less clear. A preaccretionary origin of hydrous phases in Ca-Al-rich inclusions (CAIs) has also been suggested by several scientists. In CI chondrites, no strong indications for such processes have been found, mainly due to heavy brecciation and severe parent body alteration.     

New lunar meteorite NWA 10986: A mingled impact melt breccia from the highlands—A complete cross section of the lunar crust

Northwest Africa (NWA) 10986 is a new mingled lunar meteorite found in 2015 in Western Sahara. This impact melt breccia contains abundant impact melt glass and clasts as large as 0.75 mm. Clasts are predominantly plagioclase and pyroxene‐rich and represent both highland and basalt lithologies. Highland lithologies include troctolites, gabbronorites, anorthositic norites, and troctolitic anorthosites. Basalt lithologies include crystalline clasts with large zoned pyroxenes representing very low titanium to low titanium basalts. In situ geochemical analysis of minerals within clasts indicates that they represent ferroan anorthosite, Mg‐suite, and gabbronorite lithologies as defined by the Apollo sample collection. Clasts representing magnesian anorthosite, or “gap” lithologies, are prevalent in this meteorite. Whole rock and in situ impact glass measurements indicate low incompatible trace element concentrations. Basalt clasts also have low incompatible trace element concentrations and lack evolved KREEP mineralogy although pyroxferroite grains are present. The juxtaposition of evolved, basaltic clasts without KREEP signatures and highland lithologies suggests that these basaltic clasts may represent cryptomare. The lithologies found in NWA 10986 offer a unique and possibly a complete cross section view of the Moon sourced outside of the Procellarum KREEP Terrane.     

Differentiation processes in FeO‐rich asteroids revealed by the achondrite Lewis Cliff 88763

Olivine‐dominated (70–80 modal %) achondrite meteorite Lewis Cliff (LEW) 88763 originated from metamorphism and limited partial melting of a FeO‐rich parent body. The meteorite experienced some alteration on Earth, evident from subchondritic Re/Os, and redistribution of rhenium within the sample. LEW 88763 is texturally similar to winonaites, has a Δ¹⁷O value of ?1.19 ± 0.10‰, and low bulk‐rock Mg/(Mg+Fe) (0.39), similar to the FeO‐rich cumulate achondrite Northwest Africa (NWA) 6693. The similar bulk‐rock major‐, minor‐, and trace‐element abundances of LEW 88763, relative to some carbonaceous chondrites, including ratios of Pd/Os, Pt/Os, Ir/Os, and ¹⁸⁷Os/¹⁸⁸Os (0.1262), implies a FeO‐ and volatile‐rich precursor composition. Lack of fractionation of the rare earth elements, but a factor of approximately two lower highly siderophile element abundances in LEW 88763, compared with chondrites, implies limited loss of Fe‐Ni‐S melts during metamorphism and anatexis. These results support the generation of high Fe/Mg, sulfide, and/or metal‐rich partial melts from FeO‐rich parent bodies during partial melting. In detail, however, LEW 88763 cannot be a parent composition to any other meteorite sample, due to highly limited silicate melt loss (0 to <<5%). As such, LEW 88763 represents the least‐modified FeO‐rich achondrite source composition recognized to date and is distinct from all other meteorites. LEW 88763 should be reclassified as an anomalous achondrite that experienced limited Fe,Ni‐FeS melt loss. Lewis Cliff 88763, combined with a growing collection of FeO‐rich meteorites, such as brachinites, brachinite‐like achondrites, the Graves Nunataks (GRA) 06128/9 meteorites, NWA 6693, and Tafassasset, has important implications for understanding the initiation of planetary differentiation. Specifically, regardless of precursor compositions, partial melting and differentiation processes appear to be similar on asteroidal bodies spanning a range of initial oxidation states and volatile contents.     

Compositional constraints on the genesis of diogenites

Abstract– We have done bulk rock compositional analyses (INAA, ICP‐MS) and petrologic study of a suite of diogenite meteorites. Most contain orthopyroxenes with mg#s of 70.6–79.0. Meteorite Hills (MET) 00425 is magnesian (mg# of 83.9). Lewis Cliff (LEW) 88011 contains orthopyroxene grains of varying mg# (76.3–68.6). Queen Alexandra Range (QUE) 93009 (orthopyroxene mg# 70.6) contains coarse‐grained noritic clasts (plagioclase An_84.7–88.3), and is rich in incompatible trace elements. It has Eu/Eu* < 1, indicating that cumulate norites do not dominate its trace element inventory. Queen Alexandra Range 93009 may be transitional between diogenites and magnesian cumulate eucrites. Lewis Cliff 88679, a dimict breccia of harzburgite and orthopyroxenite, has anomalously low concentrations of highly incompatible elements (e.g., Nb, La, Ta, U) compared to other diogenites, but is similar to them in less highly incompatible elements (e.g., Y, Zr, Yb, Hf). It is unlikely that this characteristic reflects a low proportion of a trapped melt component. The highly incompatible elements were likely mobilized after impact mixing of the two parent lithologies. Graves Nunataks 98108 shows an extreme range in Eu/Eu* attributable to the heterogeneous distribution of plagioclase; one sample has the lowest Eu/Eu* among diogenites. We find no compelling evidence to support the hypothesis that diogenite parent magmas were contaminated by partial melts of the eucritic crust. We posit that subsolidus equilibration between orthopyroxene and minor/trace phases (including phosphates) resulted in preferential redistribution of Eu²⁺ relative to Eu³⁺ and other rare earth elements, and results in anomalously low Eu/Eu* in samples leached in acids that dissolve phosphates.     

Mineralogy and petrogenesis of lunar magnesian granulitic meteorite Northwest Africa 5744

Lunar meteorite Northwest Africa (NWA) 5744 is a granulitic breccia with an anorthositic troctolite composition that may represent a distinct crustal lithology not previously described. This meteorite is the namesake and first‐discovered stone of its pairing group. Bulk rock major element abundances show the greatest affinity to Mg‐suite rocks, yet trace element abundances are more consistent with those of ferroan anorthosites. The relatively low abundances of incompatible trace elements (including K, P, Th, U, and rare earth elements) in NWA 5744 could indicate derivation from a highlands crustal lithology or mixture of lithologies that are distinct from the Procellarum KREEP terrane on the lunar nearside. Impact‐related thermal and shock metamorphism of NWA 5744 was intense enough to recrystallize mafic minerals in the matrix, but not intense enough to chemically equilibrate the constituent minerals. Thus, we infer that NWA 5744 was likely metamorphosed near the lunar surface, either as a lithic component within an impact melt sheet or from impact‐induced shock.     

Labile trace elements in basaltic achondrites: Can they distinguish between meteorites from the Moon,Mars, and V‐type asteroids?

Abstract— We report data for 14 mainly labile trace elements (Ag, Au, Bi, Cd, Cs, Ga, In, Rb, Sb, Se, Te, Tl, U, and Zn) in eight whole‐rock lunar meteorites (Asuka [A‐] 881757, Dar al Gani [DaG] 262, Elephant Moraine [EET] 87521, Queen Alexandra Range [QUE] 93069, QUE 94269, QUE 94281, Yamato [Y‐] 793169, and Y‐981031), and Martian meteorite (DaG 476) and incorporate these into a comparative study of basaltic meteorites from the Moon, Mars, and V‐type asteroids. Multivariate cluster analysis of data for these elements in 14 lunar, 13 Martian, and 34 howardite, eucrite, and diogenite (HED) meteorites demonstrate that materials from these three parents are distinguishable using these markers of late, low‐temperature episodes. This distinguishability is essentially as complete as that based on markers of high‐temperature igneous processes. Concentrations of these elements in 14 lunar meteorites are essentially lognormally distributed and generally more homogeneous than in Martian and HED meteorites. Mean siderophile and labile element concentrations in the 14 lunar meteorites indicate the presence of a CI‐equivalent micrometeorite admixture of 2.6% When only feldspathic samples are considered, our data show a slightly higher value of 3.4% consistent with an increasing micrometeorite content in regolith samples of higher maturity. Concentrations of labile elements in the 8 feldspathic samples hint at the presence of a fractionated highly labile element component, possibly volcanic in origin, at a level comparable to the micrometeorite component. Apparently, the process(es) that contributed to establishing lunar meteorite siderophile and labile trace element contents occurred in a system open to highly labile element transport.     

The lherzolitic shergottite Grove Mountains 99027: Rare earth element geochemistry

Abstract— We report here on an ion probe study of rare earth element (REE) geochemistry in the lherzolitic shergottite Grove Mountains (GRV) 99027. This meteorite shows almost identical mineralogy, petrology, and REE geochemistry to those of the lherzolitic shergottites Allan Hills (ALH) A77005, Lewis Cliff (LEW) 88516, and Yamato (Y‐) 793605. REE concentrations in olivine, pyroxenes, maskelynite, merrillite, and melt glass are basically comparable to previous data obtained from ALH A77005, LEW 88516, and Y‐793605. Olivine is the dominant phase in this meteorite. It is commonly enclosed by large (up to several mm) pigeonite oikocrysts. Non‐poikilitic areas consist of larger olivine grains (?mm), pigeonite, augite, and maskelynite. Minor merrillite (up to 150 μm in size) is widespread in non‐poikilitic regions, occurring interstitially between olivine and pyroxene grains. It is the main REE carrier in GRV 99027 and has relatively higher REEs (200–1000 × CI) than that of other lherzolitic shergottites. A REE budget calculation for GRV 99027 yields a whole rock REE pattern very similar to that of other lherzolites. It is characterized by the distinctive light REE depletion and a smooth increase from light REEs to heavy REEs. REE microdistributions in GRV 99027 strongly support the idea that all lherzolitic shergottites formed by identical igneous processes, probably from the same magma chamber on Mars. Despite many similarities in mineralogy, petrography, and trace element geochemistry, subtle differences exist between GRV 99027 and other lherzolitic shergottites. GRV 99027 has relatively uniform mineral compositions (both major elements and REEs), implying that it suffered a higher degree of sub‐solidus equilibration than the other three lherzolites. It is notable that GRV 99027 has experienced terrestrial weathering in the Antarctic environment, as its olivine and pyroxenes commonly display a light REE enrichment and a negative Ce anomaly. Caution needs to be taken in future chronological studies.     

Oxygen isotopic compositions and origins of calcium‐aluminum‐rich inclusions and chondrules

Abstract— Primary minerals in calcium‐aluminum‐rich inclusions (CAIs), Al‐rich and ferromagnesian chondrules in each chondrite group have δ¹⁸O values that typically range from ?50 to +5%0. Neglecting effects due to minor mass fractionations, the oxygen isotopic data for each chondrite group and for micrometeorites define lines on the three‐isotope plot with slopes of 1.01 ± 0.06 and intercepts of ?2 ± 1. This suggests that the same kind of nebular process produced the ¹⁶O variations among chondrules and CAIs in all groups. Chemical and isotopic properties of some CAIs and chondrules strongly suggest that they formed from solar nebula condensates. This is incompatible with the existing two‐component model for oxygen isotopes in which chondrules and CAIs were derived from heated and melted ¹⁶O‐rich presolar dust that exchanged oxygen with ¹⁶O‐poor nebular gas. Some FUN CAIs (inclusions with isotope anomalies due to fractionation and unknown nuclear effects) have chemical and isotopic compositions indicating they are evaporative residues of presolar material, which is incompatible with ¹⁶O fractionation during mass‐independent gas phase reactions in the solar nebula. There is only one plausible reason why solar nebula condensates and evaporative residues of presolar materials are both enriched in ¹⁶O. Condensation must have occurred in a nebular region where the oxygen was largely derived from evaporated ¹⁶O‐rich dust. A simple model suggests that dust was enriched (or gas was depleted) relative to cosmic proportions by factors of ～10 to >50 prior to condensation for most CAIs and factors of 1–5 for chondrule precursor material. We infer that dust‐gas fractionation prior to evaporation and condensation was more important in establishing the oxygen isotopic composition of CAIs and chondrules than any subsequent exchange with nebular gases. Dust‐gas fractionation may have occurred near the inner edge of the disk where nebular gases accreted into the protosun and Shu and colleagues suggest that CAIs formed.